Influence of calcium concentration on formation of tetravalent chromium doped Y3Al5O12 ceramics

Yttrium Aluminium Garnet (YAG) ceramics doped with chromium were prepared by solid-state reactive sintering in a vacuum. The influence of the charge compensator Ca2+ concentration on microstructure, optical properties and efficacy of Cr3+ oxidation to Cr4+ under air annealing was investigated. A non-monotonic dependence of these features on the amount of CaO as an additive was found. The changes in ceramic transparency and microstructure were explained considering the interaction between CaO and Cr2O3 at the ceramic grain boundaries, which leads to a different pore evolution in distinct samples during sintering. The efficacy of the oxidation of Cr3+ to Cr4+ strongly depends on the concentration of Ca dissolved in the YAG. The calcium solubility decreases due to the higher oxygen partial pressure of the extra phases on the grain boundaries that decreases the amount of generated Cr4+ ions. Such phenomenon explains the lower concentration of Cr4+ ions in the sample with 0.8% of Ca against the one with 0.5%. The experiment shows that the ceramic with 0.5% of Ca has a better in-line transmission and a higher concentration of Cr4+ ions in comparison with samples with a different Ca concentration.